Objectives We are using the rhesus monkey as a model to further characterize the behavioral and physiological correlates of extreme BI and to investigate the brain mechanisms underlying this trait. ABSTRACT:Behavioral inhibition (BI) or in its extreme form, freezing, is an adaptive response which individuals engage in when confronted with threatening situations. Clinical research has demonstrated that in its excessive form BI is a trait marker for children that are very shy and develop into overly fearful adolescents and adults. In addition, these individuals have increased adrenal-cortical and autonomic activity when tested in nonstressful conditions. Later in life, these individuals have an increased likelihood of developing clinically significant anxiety and depression suggesting that extreme behavioral inhibition may be an early marker for later development of psychopathology. Because of its potential clinical importance, we have developed a paradigm in rhesus monkeys to assess BI. As in human children, we have found that there is marked individual variability in the tendency to engage in BI and that this is a stable trait. Also, we have found that benzodiazepine systems modulate behavioral inhibition and that in rhesus monkeys marked individual differences exist in the behavioral and physiological responses to benzodiazepines. Keywords benzodiazepine, neurobiology, development, fear, anxiety, cortisol, defense, behavior, brain, psychopathology